In general, composites are heterogeneous materials made from a combination of different matrix and reinforcing materials (primarily high-strength, high-stiffness fibers), which are subjected to interfacial compatibility issues, as well as difficulties in recycling. One promising approach for enhanced interfacial properties and improved recyclability is to use a single polymer for both matrix and reinforcement. Although the original concept of single polymer composites (SPCs) was proposed by Capiati and Porter [1] more three decades ago, the progress in SPCs processing has been rather slow. The primary hurdle is due to the proximity in melting temperature between the matrix and the reinforcement. It is well known that the melting temperature of a reinforcing fiber can be raised by increasing crystallinity and perfecting the crystalline structure. However, the resulting change in melting temperature is quite small as compared with a realistic processing temperature window and its variation in standard melt processing, such as extrusion and injection molding. As a result, the fiber would be melted or its mechanical properties would be severely degraded should a standard melt process be used for SPCs processing.
So far, the work in SPCs manufacturing has been focused almost exclusively on a hot compaction process [e.g., 2-7], where polymer fibers are compacted at a temperature very close to, but below, the polymer melting temperature so as to partially melt the fiber and fuse them into a single solid material. The major challenge in this process is the small difference, typically less than 5° or 10° C., between the feasible processing temperature and the fiber melting temperature. Within such a small temperature window, it is difficult to process the SPC under normal processing conditions without significantly annealing the fiber. It is known that polymer fibers annealed at a temperature close to their melting temperature results in a much reduced modulus toward that of the unoriented polymer [5]. Furthermore, the above method relies on the creation of a textile preform before hot compaction, which is not only an expensive procedure but is also limited to simple geometries. This process is also not compatible with standard polymer processing techniques for mass production, such as extrusion and injection molding.
The concept of “overheating” above the fiber melting temperature by constraining fibers was introduced in preparing SPCs [8]. Physically fixing the fiber ends can prevent shrinkage and molecular reorientation [9-11]. To a certain extent, this method can enlarge the processing temperature window. The melting temperature of the constrained PP fiber increased by about 20° C. compared to the unconstrained PP fiber [12]. The overheating behavior of constrained fibers also has been reported for PA6 (polyamide 6) and PET (polyethylene terephthalate), but only melting temperature shifts of 10° C. and 7° C., respectively, were observed [11].
To further enlarge the processing temperature window, researchers have also utilized polymers with same chemical composition but different chemical structures. Teishev et al. [13] reinforced HDPE (high-density polyethylene) matrix with UHMWPE (ultra-high molecular weight polyethylene) fibers, and the process window was enlarged to 20° C. Devaux and Cazé [14] reinforced LDPE (low-density polyethylene) with UHMWPE fibers, and the process window was further enlarged to about 40° C. Pegoretti et al. [15] prepared SPCs based on liquid-crystalline fibers, Vectran®M and Vectran®HS. These two kinds of commercial fibers have the same chemical composition but different melting point. The resulting temperature window for SPCs processing ranged from 260° C. to 285° C. Although manufacturability was greatly enhanced in these composite systems, the interfacial adhesion was found to be lower than the original SPC. Mead et al [2] studied and found that the interfacial shear strength for HDPE films embedded in an LDPE matrix is 7.5 MPa and for HDPE self-composites is 17 MPa. In more rigorous definition, composites involving polymers with same chemical composition but different chemical structures are not true SPCs.
Recently, Yao et al. [16] proposed to widen the processing temperature window utilizing the slow crystallization kinetics of some slowly crystallizing polymers such as PET and PLA (polylactide). A slowly crystallizing polymer can be supercooled into a nearly amorphous phase. This amorphous material can then be liquefied by rapidly heating to a temperature well above the glass transition temperature (Tg) but considerably below the melting temperature (Tm) and combined with high-strength fibers to form an SPC. With this approach, the processing temperature window for PET SPCs was extended to about 70° C. However, there are two competing processes occurring when an amorphous polymer is heated. In order to avoid premature crystallization before fusion, the amorphous polymer needs to be heated rapidly throughout the entire thickness. This method is limited to polymers with a relatively long crystallization half-time; it is difficult to apply it to fast crystallizing polymers, including PP, PE and PA6/66.
Thermal liquid crystal polymers (TLCPs) are known to supercool when cooled below their melting point [18,19]. Done and Baird [18] studied the rheology of liquid crystal polymers below their normal melting temperature by measuring dynamic mechanical properties. It was found that the TLCPs could be supercooled to 50° C. below their normal melting temperatures and can still be deformed. Extrusion studies on these materials were also carried out, and it was observed that in this supercooled state the polymer extrudate exhibited significant die swell. These results demonstrated that undercooled TLCP melts can be processed using normal melt processing techniques. Not only TLCPs but also typical thermoplastic polymers may be processed in a supercooled liquid state. This is supported by the typical crystallization thermograms observed in differential scanning calorimetry (DSC). For instance, PP was found to exhibit a large degree of supercooling (˜40° C.) under normal cooling rates (˜20° C./min) in DSC [20]. However, the supercooling properties of thermoplastic polymers have not been explored in SPCs processing, and a feasible melt processing technique for SPCs processing is not known.
Despite these efforts, there remains better processing protocols for SPCs, that are compatible with standard high-throughput melt processes for polymers/plastics.